1. Field of the Invention
This invention relates to a method and an apparatus for measuring the duration and width of ultrashort laser pulses in the evaluation etc. of the properties of ultrashort-pulse lasers that can be applied to ultra-high speed electronic devices, ultra-high speed photo-chemical reaction measurement apparatuses, highly time-resolved optical measurement instruments and the like.
2. Prior Art Statement
Because of their slow response speed, conventional and ordinary optical detectors cannot be used for direct photoelectric measurement of ultrashort laser pulses having widths of less than a few picoseconds (10.sup.-12 sec). In the case of ultrashort laser pulses generated from visible or near infrared lasers whose wavelengths are longer than about 400 nm, therefore, the practice has been to carry out the pulse width measurement by the autocorrelation method using the second harmonic generation in a nonlinear crystal.
Since the distance that light travels through air in one picosecond is 0.3 mm, this conventional autocorrelation method measures the width or duration of ultrashort laser pulses by replacing time with the light propagation distance. Specifically, as shown in FIG. 4, a laser pulse p to be measured is directed into an autocorrelator 1 composed of a half-mirror M1 and reflecting mirrors M2, M3 and M4. The half-mirror M1 splits the laser pulse p into two laser pulses p.sub.1 and p.sub.2. The pulse p.sub.1 is reflected by the reflecting mirror M2 and travels to a lens 2, thus traversing a prescribed spatial distance. The pulse p.sub.2 is reflected by the reflecting mirror M3 and travels to the lens 2, also traversing a prescribed spatial distance. The lens 2 focuses the pulses p.sub.1, p.sub.2 from the autocorrelator 1 into a nonlinear crystal 3.
When the distances that the pulses p.sub.1, p.sub.2 divided by the half-mirror M1 travel from M1 to the nonlinear crystal 3 are of the same length, the two pulses overlap in time in the nonlinear crystal 3. As a result, the second harmonic radiation of the incident laser light (the second harmonic wavelength being .lambda./2 where the laser wavelength is .lambda.) is generated in the nonlinear crystal 3 in the direction approximately bisecting the cross angle of the two pulses and arrives at a light detector 5 through an aperture 4.
When there is a difference between the propagation path lengths of the two laser pulses p.sub.1, p.sub.2 which is longer than that corresponding to the laser pulse duration, no temporal overlap of the laser pulses occurs in the nonlinear crystal 3, and the second harmonic radiation is not generated.
As the intensity of the second harmonic produced increases in proportion to the square of the intensity of the laser pulses incident on the nonlinear crystal 3, the pulse duration or the pulse width of the laser pulse p can be measured by translating either the reflecting mirror M2 or the reflecting mirror M3 in parallel with the optic axis and detecting and recording the intensity variation of the second harmonic radiation as a function of the position of the mirror M2 (or M3) that is moved.
This ultrashort laser pulse measurement method using a nonlinear crystal is useless when the laser wavelength is in the ultraviolet region of less than about 400 nm, because nonlinear crystals capable of producing the second harmonic radiation in a broad spectral region of less than 200 nm are not available.
Instead, therefore, there has so far been adopted a method using an arrangement such as shown in FIG. 5(a) or 5(b) for the ultraviolet laser pulse measurement, in which a cell 6 filled with a gas is inserted into the measurement system in place of the nonlinear crystal.
In the arrangement shown in FIG. 5(a), the measurement is conducted by detecting a signal of ions generated in the gas cell 6 by the absorption of multiple ultraviolet laser photons. For this purposes, a pair of electrodes 7 are disposed in the gas cell 6 for collecting the ions and the relative number of ions produced is counted.
In the arrangement shown in FIG. 5(b), the measurement is conducted by detecting a signal of fluorescent light reemitted from the gas after absorption of the multiple ultraviolet photons. Specifically, the fluorescent light passing out of the gas cell 6 is collected by a lens 9, passed through a filter 8 and detected by a light detector 5.
The ion signal or the fluorescent light signal is observed as a function of the position of either the reflecting mirror M2 or the reflecting mirror M3 of the autocorrelator 1, whereby the duration, the width or the like of the laser pulse is measured.
While the measurement method shown in FIG. 4 using the nonlinear crystal 3 is simple and enables precise measurement, it has a drawback related to the fact that the wavelength dependence of refraction index (dispersion) of a nonlinear crystal is generally large. In the measurement related to ultrashort laser pulses, therefore, the measurement gives a pulse-width value larger than the actual value if the crystal thickness is beyond an appropriate range.
While this means that, for improving the measurement precision, the thickness of crystal used has to be reduced as the laser pulse width to be measured becomes shorter, as a practical matter it is very difficult to make thin crystals having a good surface parallelism and an extremely small surface roughness. Moreover, a thin crystal weakens the second harmonic signal and is often unable to produce a detectable signal of adequate magnitude.
In the measurement of ultraviolet ultrashort laser pulses by detecting the ion signal in the manner shown in FIG. 5(a), the measurement precision is liable to be seriously degraded by dielectric breakdown of the cell gas, recombination of electrons and ions and the like. The method thus has the disadvantage of requiring various complex procedures in selecting the gas and the gas pressure, optimizing the intensity of the laser beam to be measured, optimizing the voltage applied on the electrodes for ion collection and otherwise determining the best measurement conditions.
The ultraviolet ultrashort laser pulse measurement by observing fluorescent light in the manner shown in FIG. 5(b) has problems in that, depending on the incident laser wavelength, the sample gas has to be selected for ensuring that the multiple photons are resonantly absorbed by the gas and the reemission of light from the gas is easy to be detected. For these reasons, the method cannot be applied to the measurement of ultraviolet ultrashort laser pulses of arbitrary wavelength.
In addition, the cell gas pressure gas has to be optimized for preventing loss of the absorbed light energy by collisions between the gas atoms or molecules, and the incident laser intensity has to be optimized for preventing secondary excitation or deexcitation.
FIG. 6 shows an example of the results obtained by the method of FIG. 5(b), where ultrashort KrF excimer laser (wavelength: 248 nm) pulses were directed into molecular xenon (Xe.sub.2) gas sealed in a cell and the fluorescent light emitted was detected with a light detector. Since either of the two beams into which the laser beam is split produces a fluorescent signal by itself, it is impossible to avoid a background signal C from overlapping the signal B to be detected and a resulting decrease in the signal to noise ratio of the autocorrelation signal for the pulse measurement.
An object of this invention is to overcome the aforesaid shortcomings of the prior art methods by providing a method for measuring ultrashort laser pulses which enables simple, high-precision measurement of ultrashort laser pulses without using a nonlinear crystal or a cell filled with a gas.
Another object of the invention is to provide an apparatus for carrying out the method.